Impedance-matching device



G. F. BARNETT 2,516,944

IMPEDANCE-MATCHING DEVICE Filed Dec. 18, 1947 l/Qifa .l/I//f//l///f l//ll/I/ll/lI/l//52)Illu JNVENTOR. GUY E BAR/V577' Patented Aug. l, 1950 Philco Corporation, Philadelphia, Pa.; a corporation of Pennsylvania application December 1s, 1947, seriallvczoasso "The ,invention herein described and claimed relates'to an improvement in wide-bandmicro- .wave systems. More particularly, the invention provides ineans capable of effecting a substantiallyfreilectionless transition from a coaxial 'inode ofwave `propagation ,to a helical mode, for thereverseQover avery wide Iband of high radio ,iredlienciesI f I I While the invention is'adapted'to be used in `various applications, the invention may be em- Iployed to particular advantage as a means for couplingatraveling-wave amplifier tube to an `exlternal,coaxial-line transmission system.

`TheAtraveling-wave `tube is a comparatively ,new'ty V`e`fo`f vacuum' tube intended for use in the loommunication's art. The structure,` theory lor ,operatin,`utilization` and. advantages of vthe traveling-wave tube', particularly as an amplier ifof micro aves over a wide band of frequencies, lhavebeendescribed in recent publications including Electronics, November 1946, pages 90-92; Wireless World, November 1946, pages 369-372; jlell Laboratories Record, December 1946, pages A39, Aw;,"`fnd Proceedl gs of 1the I. R, E., Febru- 'ary"19'47;"pages10S-127. *is described in the above publications, the 'traveling-wave tubeA includes a helical trans,- missiondine, or,` helix, `whose function will be readily'vunderstood from a brief description of 4the operationof the tube.,` The tubes operation ,depends'upon theinteraction between the electric 'field ofan electromagnetic waveand 'an electron stream, both ofwhich are travelingin the same direction at 'approximately the same velocity'. 4lllhile an electromagnetic wave ordinarily travels jat about the Vspeed of light, electrons ordinarily travel at' a much` slower speed. An extremely high voltage, of the order `of ten million `v`olts, would berequired to project an electron at a velocity Aapproaching that of light. In order thatthefvelocity "of the wavefmay be approximately the same as, and preferably 'a little slower them the velocity` of electrons 4projected by a voltagzefof., reasonable magnitude, there is in, eluded, `within the evacuated envelope'of vthe traveling- Wave tube,` a helical transmission line', or' helixLwhose principal function is to retard rtheaxial progress ofthe wave. v It will be understood that the wave continues to travelat'approx- `inclately the veloeltyof light but that it follows 'the turns'fofftlle" l1` `el 2fa1,1 cl4 in Jso vdoing travels a path substantiallylcrtger `than the axial length cf the helix. In some of the traveling-wave tubes now being tested ,and used experimentally, Vthe Vhelical path* thirteen times 'longer than 4the axis or 4 claims. (C1. 17e-44) the helix. In such case, the axial electric eld travels down the helix at about one-thirteenth the speed of light. This is substantially the same as the velocityof electrons projected by a potenoperating band being largely determined, at the present state of the art, by the impedance matches at the input and output ends of the helix, that is, by the impedance matches at the ljunctions between the ends of the tubes helical transmission line and the external transmission `line system.

t In accordance with the present invention, modifications are provided, both to the prior art helix and to the prior art coaxial-line terminating section, which permit the traveling-wave tube to be coupledinto a microwave coaxial transmission line system without substantial rellection occurring, at eitherV the input or output junctions, over a very wide band of ultrahigh and superhigh frequencies.

In accordance with the broader aspects of the 'present invention, improved transition means are provided for effecting `transfer of electromagnetic wave energy from a coaxial line to a helical line, or from a helical line to a coaxial line, whether or not a traveling-wave tube is involved. It is'an object then of this invention to provide `improved means for electing transition lfrom a coaxial to a helical mode of wave propagation, Vor the reverse,i. e., from a helical to a coaxial mode of propagation. f Itis another object of this invention to provide improved means for accomplishing transfer of 'electromagnetic wave energy from a coaxial line to` a helical line, or the reverse. It isanother object of this invention to provide improved means for accomplishing transfer ofelectromagnetic `wave energy from a coaxial line to a helical line, or the reverse, without incurring substantial reflection at any frequency within `a'vviole band of ultrahigh and superhigh operating frequencies. l Another objectief this invention is to provide improved'means for coupling a helical transmission linegdirectly to a coaxial transmission line system,l without employing frequency-sensitive impedance-'matching devices.

Another object is to provide, for a traveling 4vvaveftube,` improved input and output connec- `tionssuitable fo'r connecting the external coaxial :system to and-from the helix of the tube.

tion from the radial electric'jeld of 'thecoaidal mode of 'wave propagation to the axial or turnto-turn electric eld of theY helical mode.` bserve that as the center conductor 23av ofL the. coaxial line begins to'spiral in fturns of constantly increasing diameter, the spiral conductor moves closer and closer to the outercylindrical conductor 32, and unless some form of compensation is provided, the capacitance loef-` tween the inner spiraling conductor and the outer` cylindrical conductorwould steadily increase as the diameter of the spirals increased. the structure shown in the drawing, the necessary compensation is provided by omitting portionsof the outer conductor 32 at equally spaced points on the perimeter. The portions omitted are increasingly large 4in the direction of the helix and reach a maximum `at the extremity 33 of the outer conductor 32, i. e., at the point where the spiraling inner conductor is just about to attain maximum diameter. Here the outer conductor comprises but four equi-spaced points of conductive material.

The transition means employed at the output end of the traveling wave tube, i. e., the means employed to connect the helix in stub vI9 with the outgoing coaxial transmission line 30, is identical in construction to that employed at the input end of the traveling-wave tube, and just described above. The corresponding parts may be readily seen in the drawing. It is, therefore, believed to be unnecessary to further describe the output connection.

The length of the tapered portions of the helix, and of the crowned portions of the coaxial terminating section, at both the input and output ends, may be of the order of five wavelengths long, and preferably longer. In general, the longer the lengths of the tapered portions of the helix, and of the crowned portion of the coaxial outer conductor, the smoother the transition will be.

I have ascertained that, by means of the construction shown in the drawing and described above, an electromagnetic wave, having a coaxial mode of propagation, may be applied to the lnput end of a helical transmission line without substantial wave reiiection occurring at any of the frequencies of an extremely wide band of ultrahigh and superhigh freqeuncies, e. g., a band oi the order oi several thousand megacycles or more Wide. And, by the use of similar construction, the amplified Wave may be removed from the output end of the tubes helical transmission line and applied to the outgoing coaxial transmission line without incurring substantial reflections over a band of similar width.

In my experiments, using relatively crude mechanical construction, the voltage standing wave ratio at the center of the band, for a band of frequencies extending from 2500 to 3500 megacycles, was only 1.15, and at the edge of the band was only 1.30. The standing wave ratio at frequencies outside the 250G-3500 mc. band should be correspondingly low, for the transition means of the present invention includes no resonant elements.

The discovery of means capable of coupling together a coaxial and helical transmission liner without experiencing substantial reflection of.'

Wave energy, is of particular importance in the:

occurred at some of the frequencies', either irisideor outside, of the intended operating band. To prevent oscillations, the prior art travelingwave tubev has employed a helix constructed of material `having such a high attenuation constantthat energy, which is reflected at any particular frequency, will be attenuated to an extent greater, ordinarily, than the gain realized from the tube at that same frequency. The use, in the prior art traveling-wave tube, of a helix of high attenuation necessarily reduces the R.F. power level at the output of the tube and hence lreduces the gain and eiciency of the tube. 'I'he present invention, by eliminating substantial reflection of wave energy at substantially all frequencies of an extremely wide band, eliminates the necessity of employing a high attenuation helix.- Consequently, the gain and eiciency `of the tube is increased materially.A

`Ihave described my improved transition means as particularly applicable to a traveling-wave tube,'since important advantages are derived from such application. It is to be understood, however, that the improved transition means of the present invention may also be employed in other applications. In general, the invention may be employed'wherever it is desired to connect together a coaxial and helical transmission line without incurring substantial reflection of wave energy at substantially any frequency of a very wide band of high frequencies. For example, the helix, instead of being a part of a traveling-wave tube, may be employed as a matched delay line. Or the helix may be employed as'a matched attenuator or power absorber, or as a power measuring device. Other applications will readily occur to those skilled in the art.

Having described my invention, I claim:

1. In a microwave system; a coaxial transmission line comprising an inner conductor and a hollow outer cylindrical conductor; a helical transmission line comprising a helix of preselected diameter; and transition means for interconnecting said coaxial and helical transmission lines, said transition means comprising: an outer hollow cylindrical conductor having one end portion V-notched to form a multi-pronged crownlike section; and an inner conductor coaxially aligned with said outer conductor, said inner conductor including a cone-shaped helix terminating at the small-diameter end in a substantially straight coaxially-aligned conductor and terminating at the large-diameter end in a helical turn whose diameter is substantially equal to said preselected diameter of said helical transmission line, said cone-shaped helix of said inner conductor of said transition means being substantially coextensive with said multi-pronged crown-like section of said outer conductor.

2. In a microwave system; a traveling-wave tube including a helix having a mid-portion comprised of helical turns of preselected diameter and having at each end a cone-shaped portion comprised of helical turns of decreasing diameter terminating in a substantially straight conductor coaxially aligned with the axis of said cone-shaped end portion; external input and output coaxial transmission lines each having an inner conductor and an outer hollow cylindrical conductor; and means for connecting said coaxial transmission lines to opposite ends of said helix, said connecting means at each end com prising the connection of said inner conductor of said coaxial line to said straight conductor and the extension of decreasing portions of the outer 'conductor oisaid eoaxialiine towerdsaid helix beyond, the` point` where. Said inner coriductor and said straightconduetor are connected.

3.. I,n a microwave system; a traveling-wave tube including a helix having` a midfportion conn prised of helical turns 0ipreselected" diameter and having ateaelo end acme-shaped portion comprised of helical turns of decreasing dia'rnf eter terminating in a substantially straight eenductor coaxially aligned with said cone-shaped end portion; external inputand output coaxial transmission lines each `havingv an inner con-A i duetor and an enter hollow cylindrical Conductor; and meansl for connecting said coaxial transmission linesto opposite ends o f said helix, said connecting means at each end comprising means for Connecting .said inner conduotor oi Said coaxial linek to Seid straight conductor and for extending the outer conductor of said coaxial line toward saidhelix beyond the point Where said inner conductor and said straight conductorl are connected, said extended portion of said outer conductor being` substantially coextensive with said cone-sha1oed end portion of said helix, said extended portion being so constructed that While the cross-sectional .diameter is substantially constant the conductive area per unit length decreases in the direction of said helix.

4. In a microwave system; `a traveling-wave tube including a helix having a mid-portion cornprised of helical turns Vof `sulostantially constant preselected diameter and having at each end a cone-shaped portion comprised of helical turns of decreasing diameter terminating in a substantiellgstraigiit @munter ,ooexielly aligned wit!1 thev axis oteaid ,wle-Shaped endipor. oil;K4 ex te ,al input andoutnut eoaxialtanisxission lines eaQh, klaar-ingA an: inner conductor and an outer hoilowqoyiiridrioalf conductor.; and means.. for cori-y ri,.tiiie, sa `,coaxial transmission linesl to, opposite encleof said he1ix..said connecting means 2.42 titoliA end comprising the connection of. said inner .ooxidiiotor "off- 4said coaxial line, to. said Straight eoiidetor and the 'extension of.decreas-V ine portions of; said outer conductor of said oo- @filialv liner toward, Said helixA beyond the Ypoint e. said inner. conductor andlseidstiaieht oon-` diiotoit: are eoiiiie'Glied,` said extended. poriinsof said enter; conductor forming a multi-pronged structure substantially Coextensive with said ooneeshapedfeiid nortionof said helix.

itErjERENoissi Cirri.)

The follow-ine; reierenees are-o: record in the le of this patent: 3

UNITED STATES- PATENTS 

